1. Field of the Invention
The present invention relates to an apparatus for and a method of recording information on or reproducing information from a recording medium and, more particularly, to an apparatus for and a method of recording information on or reproducing information from a plurality of disk-shaped recording mediums having different substrate thicknesses.
2. Description of the Related Art
Compact discs (CDs) have been widely used as a recording medium from which recorded information is reproduced with light. In recent years, a new recording medium, known as digital video disc (DVD), has been used for digitally recording video images over a long period of time.
For reading recorded digital information from an optical recording medium, a laser beam is applied to the optical recording medium, light reflected from the optical recording medium is detected, and the level of the reflected light is converted into binary data.
FIG. 1 of the accompanying drawings shows an optical pickup device for use with a CD. As shown in FIG. 1, a laser diode (LD) 1 emits a laser beam having a wavelength of 780 nm. The laser beam emitted from the LD 1 is divided into a plurality of laser beams, e.g., three laser beams, by a grating 2. One of the three laser beams is used to read recorded information and to control the optical pickup device in focusing servo operation. The remaining two laser beams are used to control the optical pickup device in tracking servo operation. The three laser beams will also be referred to collectively as light.
A beam splitter 3 that comprises a transparent planar plate reflects the laser beams from the grating 2 toward an objective 4. Light (converged light) reflected from a CD 10 and transmitted through the objective 4 passes through the beam splitter 3 toward a photodiode (PD) 5 that serves as a photodetector. While the reflected light is passing through the beam splitter 3, the reflected light is given astigmatism by the beam splitter 3.
The objective 4 converges the laser beams onto an information recording layer 12 that comprises minute pits on the CD 10. The objective 4 also converges light reflected from the information recording layer 12 of the CD 10 through the beam splitter 3 onto the photodiode 5.
The larger the numerical aperture (NA) of the objective 4, the larger the angle through which the objective 4 converges light into a smaller spot. In FIG. 1, the objective 4 has an NA of 0.45.
The photodiode 5 detects the returning light reflected from the CD 10. Since the laser beam emitted from the LD 1 is divided into three laser beams, the photodiode 5 has three corresponding photodetector units. One of the photodetector areas serves to detect the laser beam that is used to read recorded digital information. The remaining two photodetector areas serve to detect the two laser beams for tracking servo control. Specifically, based on the difference between the optical energy quantities of the two tracking laser beams, the objective 4 is controlled to apply the laser beam used to read recorded digital information to a predetermined track on the CD 10 in tracking servo operation.
Since the light reflected from the information recording layer 12 and applied to the photodiode 5 passes as converged light through the beam splitter 3, the light is subject to astigmatism. The objective 4 is controlled in focusing servo operation based on the astigmatism thus produced.
The CD 10 has a transparent substrate 11 having a thickness t of 1.2 mm with the information recording layer 12 disposed thereon and a protective film 13 disposed on the information recording layer 12. The laser beams from the LD 10 are converged by the objective 4 and pass through the transparent substrate 11 to the information recording layer 12 that has minute pits representative of recorded information. When the laser beams are applied to pits, they are diffracted, causing the returning light that is reflected by the recording medium and applied to the photodiode 5 to be reduced in intensity. When the laser beams are applied to a pit-free area of the information recording layer 12, they are reflected, and hence the returning light has a high intensity. The returning light from the CD 10 is detected by the photodiode 5, which converts higher and lower intensities of the returning light into respective binary levels of "1" and "0", thereby reading the digital information recorded as pits on the CD 10.
While the objective 4 is being thus controlled in tracking and focusing servo modes, the laser beams are applied to a given position on the CD 10, and the returning light is detected to read the recorded digital information from the CD 10.
FIG. 2 of the accompanying drawings illustrates a digital video disc (DVD) 20 that has been proposed recently. The DVD 20 has digital information recorded in a double-sided structure, whereas the CD 10 has digital information recorded in a single-side structure. Specifically, the DVD 20 includes a first disc member comprising a substrate 21, an information recording layer 22 disposed on the substrate 21, and a protective film 23 disposed on the information recording layer 22, and a second disc member comprising a substrate 31, an information recording layer 32 disposed on the substrate 31, and a protective film 33 disposed on the information recording layer 32, the first and second disc members being bonded to each other through the protective films 23, 33. Therefore, the DVD 20 is symmetrical with respect to a median plane thereof.
Since digital information is recorded with high density on the DVD 20, the substrates 21, 31 are thinner than the substrate 11 of the CD 10 in order to minimize skews and errors of substrate thicknesses. Specifically, while the substrate 11 of the CD 10 has a thickness of 1.2 mm, each of the substrates 21, 31 of the DVD 20 has a thickness of 0.6 mm. The length of and intervals between the pits on the DVD 20 are smaller than those of the CD 10.
Inasmuch as the recording density of the DVD 20 is greater than the recording density of the CD 10, an LD 41 of an optical pickup device for use with the DVD 20 emits a laser beam having a shorter wavelength of 650 nm than the LD 1 of the optical pickup device for use with the CD 10. The optical pickup device for use with the DVD 20 has other components including a grating 42, a beam splitter 43, an objective 44, and a photodiode (PD) 45 that are identical to those of the optical pickup device for use with the CD 10.
However, because the DVD 20 has pits smaller than the CD 10 due to the larger recording density, the objective 44 has a numerical aperture (NA) of 0.6, which is greater than the objective 4 (NA=0.45) of the optical pickup device for use with the CD 10. The objective 44 with the larger numerical aperture is capable of converging a laser beam into a smaller spot to read smaller pits.
As described above, the CD 10 and the DVD 20 are structurally different from each other. Usually, therefore, it is necessary to use different optical systems (optical pickup devices) for reading recorded information from the CD 10 and the DVD 20.
If the optical pickup device for use with the DVD 20 is applied to the CD 10, for example, as shown in FIG. 3 of the accompanying drawings, then since the optical pickup device for use with the DVD 20 is designed to read the recorded information from the DVD 20 under optimum conditions, it suffers spherical aberration due to the difference between the thicknesses of the substrate 11 of the CD 10 and the substrates 21, 31 of the DVD 20 and the difference between the numerical apertures of the objectives 4, 44 when reading the recorded information from the CD 10.
For example, when a CD whose substrate has a thickness of 1.2 mm is played back using an objective having a numerical aperture of 0.6 which is optimized for a DVD whose substrate has a thickness of 0.6 mm, the amount of spherical aberration that is produced reaches 3.6 .mu.m in terms of a fourth-order Seidel spherical aberration coefficient W.sub.40. If this amount of spherical aberration is expressed by a root-mean-square value, it is 0.268 rms.mu.m (which is 0.412 rms.lambda. if normalized at a wavelength .lambda. of 650 nm). Generally, all optical systems for use with optical discs are required to have the sum of root-mean-square values of aberrations equal to or smaller than the Marechal's criterion of 0.07 rms.lambda.. Therefore, it is difficult to accurately read the recorded information from the CD 10 with the optical system arrangement shown in FIG. 3.
It has been proposed to adjust the numerical aperture of an objective to the types of different recording mediums for making an optical pickup device for use with a DVD applicable to a CD, as disclosed, for example, in Japanese patent application No. 6-277400 (which corresponds to copending U.S. patent application Ser. No. 08/555,339).
FIGS. 4 and 5 of the accompanying drawings show an optical system based on the principles of the above proposal. As shown in FIGS. 4 and 5, the optical system comprises a diaphragm 51, an actuator 53 for actuating the diaphragm 51, and a sensor 52 for detecting the type of recording medium which is used, in addition to the optical pickup device for use with a DVD, as shown in FIG. 2.
The sensor 52 detects the type of recording medium used, and the actuator 53 actuates the diaphragm 51 based on a detected signal from the sensor 52. Specifically, for reading the recorded information from the DVD 20, the actuator 53 actuates the diaphragm 51 to increase the opening thereof until the numerical aperture of the objective 44 becomes 0.6, as shown in FIG. 4. For reading the recorded information from the CD 10, the actuator 53 actuates the diaphragm 51 to reduce the opening thereof until the numerical aperture of the objective 44 becomes 0.45, as shown in FIG. 5. The opening of the diaphragm 51 is thus reduced to minimize the spherical aberration (the fourth-order Seidel spherical aberration coefficient W.sub.40 is proportional to the fourth power of the numerical aperture NA) for reading the recorded information from the CD 10.
However, because the mechanical diaphragm 51 is newly added, the optical system is made up of an increased number of parts, expensive to manufacture, and large in size and complex in structure. Since the diaphragm 51 is mechanically operated, it is not resistant to vibrations, cannot operate quickly, and tends to cause a fault in the optical system.